1. Field of the Invention
The present invention relates to a method of controlling reactive sputtering deposition of transparent metal oxide films on substrates, and further, to the formation of reproducible uniform films of indium oxide or titanium oxide.
2. Discussion of the Technical Problem
Methods are known in the art of thin films for producing a titanium oxide-based transparent electroconductive coating. For example, U.S. Pat. No. 3,698,946 to Kaspaul et al discloses coatings comprising a first layer of titanium monoxide, a second layer of copper, silver, gold, platinum, or tin, and a third layer of titanium monoxide. The coated articles are useful as photodetectors, light emitting devices, image converters and image amplifiers. While the coated articles are described as transparent and electrically conductive, the transmittances of 38 to 76 percent and resistances of 1600 to 200,000 ohms per square are unsuitable for certain applications such as motor vehicle windows which require a high transmittance, preferably 80 percent or higher, and very low resistance, preferably less than 10 ohms per square, to develop useful amounts of heat with available generator voltages. In addition, the titanium monoxide imparts a distinct color to transmitted light.
U.S. Pat. No. 3,962,488 to Gillery teaches a method for making a colorless, highly transparent coating which also has excellent conductivity, the resistance being less than 10 ohms per square. Gillery discloses that the substitution of titanium dioxide for the monoxide of Kaspaul eliminates the color and transparency problems. However, direct deposition of titanium dioxide is incompatible with the intermediate conductive layer. An intermediate silver film, for example, which is initially continuous and highly conductive becomes discontinuous, resulting in a marked increase in resistance and decrease in transmittance in less than 24 hours. Gillery's invention involves depositing the titanium oxide layers as TiO.sub.x wherein x is greater than 1.3 but less than 1.7. While the coating may initially be somewhat colored, it becomes colorless as the titanium suboxide is oxidized upon exposure to a normal atmosphere of air or when subjected to the conditions of lamination.
U.S. Pat. No. 4,194,022 to Gillery teaches a method of controlling the rate of oxidation of a film of titanium suboxide in the above-described coating by treating the coating with a vapor of an oil, wax, heavy organic alcohol, or amine prior to exposure to an oxidizing atmosphere. Such a method was preferred because uncontrolled oxidation of the titanium suboxide film may jeopardize the continuity of the adjacent electroconductive film.
While useful in producing desirable electroconductive coatings by evaporation techniques, the previously discussed techniques have limitations associated therewith which would preferably be avoided. For example, the techniques required both a deposition step and a subsequent oxidation step. Further, the treatment of the coating with an oxidation-retarding medium may provide to be inconvenient. Thus, previously utilized techniques for making a titanium oxide-based transparent electroconductive coating resulted in either undesirable optical and electrical properties, or involved inconvenient procedures. It would be desirable to eliminate such difficulties.
Methods are also known for producing a transparent, electroconductive indium oxide film. However, indium oxide films having low resistance and high luminous transmittance were previously obtainable only by deposition onto hot substrates or by deposition with subsequent heat treatment. It would be desirable to have a method of depositing a transparent electroconductive indium oxide film which could be used with both ambient temperature substrates and elevated temperature substrates.
Further, considerable difficulty has been encountered in the reactive sputtering deposition art in controlling the stoichiometry, optical characteristics, and electrical properties of deposited oxide-containing films. One source of such difficulty is the varying amounts of outgassing which occur during the sputtering process, leading to uncertainty in the composition of the sputtering atmosphere. As a result, coating uniformity and reproducibility has been less than ideal.
U.S. Pat. No. 4,113,599 to Gillery teaches a sputtering technique for the reactive deposition of indium oxide in which the flow rate of oxygen is adjusted to maintain a constant discharge current while the flow rate of argon is adjusted to maintain a constant pressure in the sputtering environment. While successful in controlling reactive deposition by conventional D.C. or R.F. processes where the substrate temperature, gas partial pressures, and gas flow rates are relatively high with respect to outgassing contributions, such a technique does not provide the preferred degree of control when utilized in reactive magnetically enhanced sputtering, where the gas partial pressures and gas flow rates are preferably relatively low. In such an environment outgassing contributions may be proportionally more significant, thus requiring more precise control over chamber conditions. It would be desirable to have a method of controlling reactive sputtering deposition which is sufficiently precise to be utilizable in magnetically enhanced sputtering techniques as well as in conventional D.C. and R. F. sputtering techniques.